The Rocky Mountain system extends from the Liard River, B. C., to Santa Fe, New Mexico, and is the dominant geological feature within the seven-state Rocky Mountain section. Five tectonic divisions based on the present structural position of the Precambrian crystalline basement exist within the system and the adjacent eastern area. The divisions are: Canadian shield; thinly covered shield; area of locally exposed basement; area of deeply buried basement; and the area where the basement is no longer recognizable.

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Precambrian rocks exposed over large areas of Canada collectively form the Canadian shield. The exposed basement is bounded in Alberta and Saskatchewan by overlying sediments. The surface upon which the overlying sedimentary rocks were deposited is warped into a major syncline the steeper limb of which lies in or near the disturbed belt of Alberta and Montana and along the Rocky Mountain front. In this syncline the basement is covered by a relatively thin veneer of sediments except for local areas such as the Williston basin. Petroleum accumulation in the area of thin sedimentary cover is controlled more by stratigraphic than by structural factors.

The area of local exposure of the Precambrian basement includes the Southern Rockies, the Wyoming basin, and part of the Middle Rocky Mountain province. Prior to deformation the basement was thinly covered by sediments. Anticlinal mountain ranges and deep local intermontane basins characterize the region. The site of deformation was localized by the Pennsylvanian structural history. Petroleum accumulation is largely controlled by geologic structure.

The Precambrian basement was deeply depressed beneath the troughs existent in central Utah, western Wyoming, eastern Idaho, western Alberta, and eastern British Columbia during late Precambrian and early Paleozoic time. Deformation of the troughs created the overthrust belt characterized by repetition of the sedimentary sequence in overthrust fault plates. Absence of Precambrian basement rocks in the thrust plates suggests that faulting is localized in the sedimentary sequence. Accumulation of petroleum have been negligible in this division.

The Precambrian basement complex is no longer recognizable in the areas of batholithic intrusion in Idaho, Montana, and British Columbia. The development of granite and granite gneiss in the Idaho batholith, age 100 MY, modified the pre-existing rocks, and substituted a new floor for subsequent geologic history.

Lineaments of primary significance are superimposed across the tectonic divisions. The lineaments have complex geologic histories in which transverse movement was probably important. The prominent lineaments are: Rocky Mountain trench, Montana lineament, Wyoming lineament, and the Walker lineament, all of which separate regions of differing geology.

Volcanically derived materials of Cenozoic age are widely spread across the Rocky Mountain system, and reflect igneous activity of great magnitude. Extensive tectonic adjustment took place by means of normal and reverse faulting both during and subsequent to the deposition of the volcanic products. The extent and magnitude of the late Cenozoic faulting are as yet incompletely understood. Critical discrimination should be made between fault systems of different ages.

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